Short-time delay fuse



E. V. SUNDT SHORT-TIME DELAY FUSE Dec. 16, 1958 Filed Dec. 23, 1954 INVENTOR. uard United States Patent O SHORT-TIME DELAY FUSE Edward V. Sundt, Wilmette, Ill. Application December 23, 1954, Serial N o. 477 ,3436

7 Claims. (Cl. 200135) The principal object of this invention is to provide an improved fuse for protecting electric circuits, which may take the place of conventional wire filament fuses, which provides for a little more lag than said wire filament fuses, which is considerably more rugged than said wire filament fuses, which provides for substantially instantaneous action upon the occurrence of shorts or the like in the circuit, which provides a short time delayed actionupon sustained overloads, which prevents needless fuse blowing on harmless overloads, which utilizes a new and improvedprinciple of operation, which is of new and improved construction, which is rugged and fool proof in operation, and which can be simply and inexpensively manufactured while maintaining close tolerances on the ratings thereof.

Briefly, the short-time delay fuse of this invention includes a fuse element having an elongated flat supporting strip formed of electrical insulating material resistant to high temperatures, a thin film of relatively low melting point fuse metal carried by one side of the supporting strip in intimate contact therewith, and a pair of longitudinally spaced apart films of relatively high melting point metal carried by the same side of the supporting strip with the film of relatively low melting point fuse metal extending therebetween and in series electrical contact therewith. Preferably, this fuse element is located within a tubular cartridge formed of electrical insulating material and provided at its ends with terminal caps, the fuse element being connected in series with the terminal caps for connection into the electric circuit being protected.

An overload in the electric circuit causes the film of relatively low melting point fuse metal to heat to the melting point thereof for breaking the electric circuit and the heating rate thereof is delayed somewhat by the intimate contact between it and the supporting strip.

The thin film of relatively low melting point fuse metal acts as its own resistance heater and, because of its nature, its resistance for a given rating is low, its heating rate may be simply and accurately calibrated and, at the same time, it is firmly supported by the supporting strip so as to be rugged in construction and foolproof in operation.

The film of relatively low melting point fuse metal and the supporting strip are notched adjacent the center thereof for decreasing the area thereof adjacent that point. This is done for the purpose of keeping the area which expands and contracts, on heating and cooling, as small as possible and accordingly there is less tendency forthe thin film to loosen away from the supporting backing strip. Control of the calibration of the fuse element may be obtained by arranging the notches closer or farther apart, thus concentrating or deminishing the heat generated as is required by the exact calibration value. The films of relatively high melting point metal and the supporting strip are notched at the ends thereof for the purpose of weakening the same.

The elongated flat supporting strip is preferably formed of thermosetting organic synthetic plastic material having 2,854,917 Patented Dec. 16, 1958 ice 10 thousandths of an inch. The thin film of relatively low melting point fuse metal preferably has a thickness within the approximate range of 5 millionths to 500 millionths of an inch and a melting point within the approximate range of to 450 degrees centigrade. Any suitable fuse. metals within these ranges may be used, as for example, bismuth, cadmium, indium, lead, selenium, tin and zincor alloys thereof. In this way, the fuse element does not. have to reach such a high temperature, and thus run. dangerously hot, before the film melts and opens on an. overload, and, also, the fuse operates at a cooler temperature when carrying its rated normal load. On the other hand, the temperature range is sufiiciently high so; that changes in ambient service temperatures do not. induce large changes in the blowing point of'the fuse. As to the thickness range of the film, the thickness of the: film is sufficiently thick to prevent the film from being oxidized away in normal service of the fuse and sufiicien'tly thin to prevent the film from flaking off on repeated heating and cooling of the fuse element.

The thin film of relatively low melting point fuse metal is in intimate contact with and adheres Well to the supporting member, and it may be applied thereto in a number of ways, such as by the processes of deposition of evaporated metals, gas plating, burnishing, peening, or chemical precipitation, the former providing exception ally good results. The films of relatively high melting point metal may be applied to the fuse element by spray coating or the like with copper or other higher temperature metal that will easily solder and these films may be considerably thicker than the films of relatively low melting point fuse metal.

Further objects of this invention reside in the details of construction of the short-time delay fuse, in the cooperative relationships between the component parts thereof, and in the manner of manufacturing the same.

Other objects and advantages of this invention will become apparent to those skilled in the art upon reference to the accompanying specification, claims and drawing in which:

Fig. l is a perspective view of a flat sheet of electrical insulating material resistant to high temperatures;

Fig. 2 is a perspective view similar to Fig. 1 but illustrating a film of relatively low melting point fuse metal applied to one side of the elongated fiat sheet in intimate contact therewith;

Fig. 3 is a perspective view similar to Figs. 1 and 2 but illustrating a pair of spaced apart films of relatively high melting point metal applied to said one side of the elongated fiat sheet in electrical contact with the film of low melting point fuse metal;

Fig. 4 is a plan view of the treated sheet illustrated in Fig. 3 and showing the manner in elements are stamped therefrom;

Fig. 5 is a plan view of one of the fuse elements so formed;

Fig. 6 is an enlarged vertical sectional view through the fuse elements illustrated in Fig. 5;

Fig. 7 is a sectional view illustrating the completed short-time delay fuse of this invention.

The time delay fuse for protecting an electric circuit is generally designated at 10 in Fig. 7 and it is formed in the manner illustrated in Figs. 1 to 5. Here, there is provided, as illustrated in Fig. 1, an elongated flat sheet of thermosetting organic synthetic plastic electrical in-' sulating material resistant to high temperatures as shown at 11. Thisflat sheet is formed from a thermosetting orwhich time delay fuse.

ganic synthetic plastic material and has a thickness within the approximate range of 5 thousandths to thousandths of an inch. A thermosetting resin such as melamine or glass fibre impregnated with a thermosetting polyester has been found to be particularly satisfactory for the purposes of this invention. They have high electrical insulating, high temperature resistant, and temperature change shock resistant characteristics. Further, these materials are not subject to are tracking when and if the fuse blows. As shown in Fig. 2 a thin film 12 of relatively low melting point fuse metal is applied to one side of the sheet 11 in intimate contact therewith, this film being applied in the manner described above and having a thickness within the approximate range of 5 millionths to 500 millionths of an inch. The melting point of the fuse metals so applied is within the approximate range of 150 to 450 degrees centigrade and the fuse metal, for example, may be bismuth, cadmium, indium, lead, selenium, tin or zinc or alloys thereof, all of which fall within this temperature range.

As shown in Fig. 3, a pair of spaced apart films 13 and 14 of relatively high melting point metal is applied to said one face of the sheet, the film 12 of relatively low melting point fuse metal extending between and electrically contacting the films 13 and 14. These films 13 and 14, which may be formed from copper or other high melting point metal, are applied in the manner described above, such as by metal spraying or the like. The film 12 forms a high resistance current path and the films 13 and 14 form low resistance current paths, the films being electrically arranged in series. This composite sheet including the backing 11 and the films 12, 13 and 14, is utilized for forming the fuse elements 15, the fuse elements 15 being stamped therefrom as illustrated in Figs. 4 and 5. In this respect the backing 11 is sufficiently flexible to be fed through a punch press in strips in order to blank out the fuse elements therefrom. In each fuse element 15 the film 12 of relatively low melting point fuse metal and the supporting strip 11 are notched adjacent the center thereof as indicated at 16 and 17 for decreasing the area of the fuse element at this point. This is done in order to keep the area which expands and contracts, on heating and cooling, as small as possible and, accordingly, there is less tendency for the film 12 to loosen away from its supporting backing strip 11. Further, control of the calibration of the fuse element may be obtained by moving the notches 16 and 17 closer or farther apart, thus concentrating or diminishing the heat generated as is required by the exact calibration value. Also, in the fuse element 15, the films 13 and 14 of relatively high melting point metal and the supporting strip 11 therefor are notched at the ends of the supporting strip 11 for weakening the same.

The fuse element illustrated in Figs. 4, 5 and 6 is preferably contained in a cartridge type of fuse including, as illustrated in Fig. 7, a tubular cartridge 20 formed of electrical insulating material, such as glass or the like, and terminal caps 21 and 22 carried by the ends of the tubular cartridge 21). The low resistance films 13 and 14 are electrically connected to the terminal caps 21 and 22 by globules of solder 23 and 24, respectively. These solder globules 23 and 24, in addition to forming the electrical connection, also operate to support the fuse element 15 centrally within the tubular cartridge 18. The soldering operation is facilitated by the end notches 18 and 19. The completed fuse may be connected into an electric circuit to be protected through the terminal caps 21 and 22.

An overload in the electric circuit causes the film 12 of relatively low melting point fuse metal to heat to the melting point thereof whereupon the film separates to break the electric circuit. Since the film 12 is in intimate contact with the supporting strip 11 the heating of the film is delayed somewhat to provide a short-time delay action. The thin film 12 of relatively low melting point fuse metal acts as its own resistance heater and, because of its nature, its resistance for a given fuse rating is low, and its heating rate may be simply and accurately calibrated by selecting the proper dimensions therefor. Upon the occurrence of a short in the electric circuit resulting in extremely high current fiow the film 12 of relatively low melting point fuse metal substantially immediately melts to break the circuit and the films 13 and 14 of relatively high melting point fuse metal may also melt adjacent the weakened portions 18 and 19 thereof additionally to break the circuit. This multiple breaking of the circuit materially reduces the tendencies of the fuse to arc. When the fuse blows, there is no are tracking on the backing strip 11 because of the nature of the materials used therefor.

The fuse of this invention may be constructed to cover substantially any desired rating range and the extent of the time delay action may be accurately regulated. For example, a fuse constructed in accordance with this invention having a one quarter amp. rating has a fuse element 1% inch long and a width of W of an inch. The film 12 is approximately .000020 inch thick and inch long. With the film 12 formed of tin the fuse produces approximately a 2 to 3 second delay at 200% rating in a 250 volt circuit.

While for purposes of illustration one principal form of this invention has been disclosed, other forms thereof may become apparent to those skilled in the art upon reference to this disclosure and, therefore, this invention is to be limited only by the scope of the appended claims.

I claim as my invention:

1. A time delay fuse for protecting an electric circuit comprising, an elongated fiat supporting strip formed of thermosetting organic synthetic plastic material having electrical insulating and high temperature resistant characteristics, a film of relatively low melting point fuse metal carried by one side of the supporting strip in intimate contact therewith, a pair of longitudinallyspaced apart films of relatively high melting point metal carried by the same side of the supporting strip with the film of relatively low melting point fuse metal extending therebetween and in series electrical contact therewith, and means including electrical terminals connected to the pair of films of relatively high melting point metal for connecting the same into the electric circuit to be protected, an overload in the electric circuit causing the film of relatively low melting point fuse metal to heat to the melting point thereof for breaking the electric circuit and the heating rate thereof being delayed by the intimate contact between it and the supporting strip.

2. A time delay fuse for protecting an electric circuit comprising, an elongated fiat supporting strip formed of thermosetting organic synthetic plastic material having electrical insulating and high temperature resistant characteristics, and a thickness range of approximately 5 thousandths to 10 thousandths of an inch, a film of relatively low melting point fuse metal carried by one side of the supporting strip in intimate contact therewith and having a thickness within the approximate range of 5 millionths to 500 millionths of an inch and a melting point within the approximate range of 1.50 to 450 de- .pair of films of relatively high melting point metal for connecting the same into the electric circuit to be protected, an overload in the electric circuit causing the film of relatively low melting point fuse metal to heat to the melting point thereof for breaking the electric circuit and the heating rate thereof being delayed by the intimate contact between it and the supporting strip.

3. A time delay fuse for protecting an electric circuit comprising, an elongated fiat supporting strip formed of thermosetting organic synthetic plastic electrical insulating material resistant to high temperatures, a film of relatively low melting point fuse metal carried by one side of the supporting strip in intimate contact therewith, a pair of longitudinally spaced apart films of relatively high melting point metal carried by the same side of the supporting strip with the film of relatively low melting point fuse metal extending therebetween and in series electrical contact therewith, said film of relatively low melting point fuse metal and the supporting strip being notched adjacent the center thereof for decreasing the area thereof adjacent that point, and means including electrical terminals connected to the pair of films of relatively high melting point metal for connecting the same into the electric circuit to be protected, an overload in the electric circuit causing the film of relatively low melting point fuse metal to heat to the melting point thereof for break ing the electric circuit and the heating rate thereof being delayed by the intimate contact between it and the supporting strip.

4. A time delay fuse for protecting an electric circuit comprising, an elongated fiat supporting strip formed of thermosetting organic synthetic plastic electrical insulating material resistant to high temperatures, a film of relatively low melting point fuse metal carried by one side of the supporting strip in intimate contact therewith, a pair of longitudinally spaced apart films of relatively high melting point metal carried by the same side of the supporting strip with the film of relatively low melting point fuse metal extending therebetween and in series electrical contact therewith, said films of relatively high melting point metal and the supporting strip being notched at the ends thereof for weakening the same, and means including electrical terminals connected to the pair of films of relatively high melting point metal for connecting the same into the electric circuit to be protected, and overload in the electric circuit causing the film of relatively low melting point fuse metal to heat to the melting point thereof for breaking the electric circiut and the heating rate thereof being delayed by the intimate contact between it and the supporting strip.

5. A time delay fuse for protecting an electric circuit comprising, an elongated fiat supporting strip formed of thermosetting organic synthetic plastic electrical insulating material resistant to high temperatures, a film of relatively low melting point fuse metal carried by one side of the supporting strip in intimate contact therewith, a pair of longitudinally spaced apart films of relatively high melting point metal carried by the same side of the supporting strip with the film of relatively low melting point fuse metal extending therebetween and in series electrical contact therewith, said film of relatively low melting point fuse metal and the supporting strip being notched adjacent the center thereof for decreasing the area thereof adjacent that point, said films of relatively high melting point metal and the supporting strip being notched at the ends thereof for weakening the same, and means including electrical terminals connected to the pair of films of relatively high melting point metal for connecting the same into the electric circuit to be protected, an overload in the electric circuit causing the film of relatively low melting point fuse metal to heat to the melting point thereof for breaking the electric circuit and the heating rate thereof being delayed by the intimate contact between it and the supporting strip.

6. A time delay cartridge fuse for protecting an electric circiut comprising, a fuse element including an elongated flat supporting strip formed of thermosetting organic synthetic plastic electrical insulating material resistant to high temperatures, a film of relatively low melting point fuse metal carried by one side of the supporting strip in intimate contact therewith, and a pair of longitudinally spaced apart films of relatively high melting point metal carried by the same side of the supporting strip with the film of relatively low melting point fuse metal extending therebetween and in series electrical contact therewith, a tubular cartridge of electrical insulating material, a pair of metallic terminal caps carried by the ends of the cartridge for electrical connection into the circuit to be protected, and solder globules supporting and mounting the fuse element in the cartridge and electrically connecting the films of relatively high melting point metal to the terminal caps, an overload in the electric circuit causing the film of relatively low melting point fuse metal to heat to the melting point thereof for breaking the electric circuit and the heating rate thereof being delayed by the intimate contact between it and the supporting strip.

7. The method of forming time delay fuse elements comprising applying to one side of an elongated flat sheet of thermosetting organic synthetic plastic electrical insulating material resistant to high temperatures in intimate contact therewith a film of relatively low melting point fuse metal, applying longitudinally to said one side of said elongated flat sheet a pair of spaced apart films of relatively high melting point metal which are in electrical contact with the film of low melting point fuse metal, and stamping from said elongated fiat sheet a plurality of transversely arranged fuse elements, each including an elongated flat supporting strip formed of thermosetting organic synthetic plastic electrical insulating material resistant to high temperatures, a film of relatively low melting point fuse metal adjacent the center thereof and a pair of longitudinally spaced apart films of relatively high melting point metal adjacent the ends thereof and in electrical contact With the film of relatively low melting point fuse metal.

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